Objects classified as brown dwarfs fail to reach what specific fusion threshold?

15 million K threshold for hydrogen fusion

Brown dwarfs represent substellar objects that failed to become true stars because they did not gather sufficient mass during their formation process. Specifically, if a collapsing core accumulates less than approximately 0.08 times the mass of the Sun, the gravitational pressure within its center is insufficient to raise the temperature high enough to initiate sustained hydrogen fusion. The required critical temperature for this primary stellar energy source is 15 million Kelvin. Lacking this ignition, brown dwarfs glow faintly, powered instead by residual heat and slow, ongoing gravitational contraction, preventing them from joining the hydrogen-burning Main Sequence.